The present invention relates generally to radiocommunication systems wherein signals are transmitted over an air interface and, more specifically, to performing mobile-assisted handoff (MAHO) measurements in such radiocommunication systems.
In wireless communications, channelization of the system bandwidth is used to provide a plurality of communications channels. The definition of a channel depends on the type of multiple access scheme employed. In frequency-division-multiple-access (FDMA), a channel refers to a subset of the total frequency spectrum available to the system. Thus, each channel is centered on a different frequency. In time-division-multiple-access (TDMA), each frequency is divided into a number of time slots and a channel refers to a particular one or more of those time slots. In code-division-multiple-access (CDMA), spreading codes are used to spread information symbols across the usable bandwidth and a channel refers to a particular spreading code used to spread and despread information symbols associated with a connection. The spreading codes consists of a sequence of values, commonly referred to as chips. Thus, a binary information symbol can be sent over the air interface in a CDMA system by transmitting either one chip sequence or another chip sequence depending upon the particular spreading code selected for that channel.
Often, hybrid systems exist which combine various access methodologies, such as FDMA/TDMA and FDMA/CDMA. In FDMA/TDMA systems there are multiple FDMA frequencies, and each frequency is used to transmit multiple time slots. In FDMA/CDMA systems there are multiple FDMA frequencies, and each frequency is used to transmit multiple codes. Hybrid FDMA/TDMA/CDMA systems are also possible.
Regardless of the multiple access scheme used, users are assigned channels for communication purposes. In cellular communication systems, users are allowed to move from one cell to the next during a call. To maintain call quality, the user is serviced from different base stations, depending on the base station(s) best able to support radiocommunications with that particular user. As a result, there are control mechanisms for handing off the call from one base station to the next, which mechanisms usually require switching from one communications channel to another.
Traditionally, these control mechanisms rely on information obtained from channel energy or power measurements made at the base stations using a scanning receiver to determine when handoffs should be performed. Since some of the first cellular systems used FDMA access schemes, the scanning receiver scanned different frequencies and made signal strength measurements. Measurements from multiple base stations were then examined at a central control point in the radiocommunication network to determine when and where handoffs should occur. These measurements were made only for one link of the communications channel, i.e. the uplink from the user to the base station.
More recently, digital cellular systems have been deployed in which measurements are also made on the downlink, i.e. on transmissions from the base station to the user. These measurements are made by the user""s equipment and communicated back to the base station via a control channel. These measurements are referred to as mobile-assisted handoff (MAHO) measurements. MAHO measurements are economically feasible because these digital cellular systems are hybrid FDMA/TDMA. Thus, the mobile station would typically receive its downlink signal during one time slot and transmit its uplink signal during another time slot. However, each TDMA frame in these systems typically has more than two time slots, e.g., six or eight time slots per frame. These other time slots are typically allocated for usage as different communication channels as described above. Thus, a mobile station which is connected in this manner to an FDMA/TDMA system will be idle for several time slots during each frame. These idle time slots are available for making MAHO measurements. Thus, the same receiver hardware in the mobile station is used both for receiving the downlink signal and for making MAHO measurements.
However, such an approach is limited to systems which have a TDMA component in their access scheme and available idle time slots for making MAHO measurements. Otherwise, for example if the receiver must continuously monitor the downlink, then a separate receiver is required for making MAHO measurements, which adds significant cost and size to the user""s terminal. Thus, there is a need for an alternative, efficient, cost-effective way of performing MAHO measurements in a wireless communications terminal.
The present invention provides an efficient method for performing MAHO measurements in a wireless terminal. According to exemplary embodiments, the received signal is split at a point in the signal processing where the entire system band is available for high-speed digitization. A snapshot of this signal is digitized and then processed digitally to provide channelization and signal strength information. This information is then reported to the system for usage in making handoff determinations.
According to one exemplary embodiment of the present invention, a signal splitter is inserted downstream of the intermediate frequency generator in the receive signal processing path. One copy of the signal is processed conventionally to provide the information signal to the terminal""s processor. Another copy of the signal is digitized, channelized and measured for signal strength (or bit error rate) to provide MAHO measurement information which is then transmitted back to the base station.
Various techniques for processing the signal to obtain the MAHO measurement information are described. According to one exemplary embodiment, a channelizer is provided which separates the channel (or channels) to be measured from other channels present in the received signal. The particular signal processing techniques applied in the channelizer will depend upon the multiple access technique associated with the system. The magnitude of the received signal is then determined and accumulated, providing an estimate of the signal strength for the selected channel(s).
According to another exemplary embodiment, a Fast Fourier Transform (FFT) processor is used to produce frequency channelized data streams for a plurality of frequencies. Depending upon the access methodology used, extraction devices may follow the FFT processor to support, for example, TDMA and CDMA access components. As in the previously described embodiment, the magnitude of the received signal is then determined and accumulated, providing an estimate of the signal strength for the selected channel(s).